Phenylalanine hydroxylase was found in extracts from autoptates of human liver, kidney, myocardium, small intestine, lung and spleen. In all the tissues studied, except of spleen, the antigen was detected by immunoelectrophoresis with monospecific antiserum against human liver phenylalanine hydroxylase. As shown by immunoblotting carried out after electrophoresis under denaturating conditions, the antigen, observed in various tissues, exhibited similar electrophoretic mobility which was very close to that of the enzyme purified from human liver tissue. Molecular mass of revealed antigen was estimated 55-57 kDa. Coincidence of immunochemical and chemical properties of the protein suggested that the antigen, detected in the tissue extracts, was a product of phenylalanine hydroxylase gene expression. The antigen concentration did not correlate with the content of albumin in tissue extracts, thus demonstrating that the revealed antigen did not occur in these preparations with blood contaminations. Content of the antigen in the tissue extracts studied was (ug per g): liver - 1500-1900, kidney - 300-575, brain - 20-40, myocardium - 85-105, lung - 40-125, small intestine - 45-70, spleen - 0-12.